1. Field of the Invention
The present invention relates to an endoscope that employs an image pickup device having only a vertical optical black pixel region.
2. Description of the Related Art
Conventionally, endoscopes provided with an image pickup device have been widely used in medical fields and industrial fields. Further, a technology for configuring an endoscope system has been known in which a signal processing apparatus, what is called a processor, is detachably connected with an endoscope to perform various kinds of signal processing related to the endoscope.
In addition, an image pickup device such as a CCD image sensor which is mounted to the endoscope of such a type includes, as is known, an effective pixel region where light is actually received and electric charge is obtained by photoelectric effects, and an optical black pixel region (OB region) provided around the effective pixel region, for performing offset detection of the electric charge including dark current.
Generally, the pixels in the image pickup device have characteristics in which, when the respective pixels have heat, electric charge is accumulated even if the pixels are not irradiated with light, and what is called dark current noise is generated.
The above-described optical black pixel region is arranged in the peripheral portion of the image pickup device in an optically-masked state. As a result, the optical black pixel region is not irradiated with light. Therefore, detection of the black level in the optical black pixel region enables the detection of only offset information of the electric charge including dark current noise and the like, and subtraction of the black level from the pixel information of the effective pixel region enables generation of an image pickup signal with the signal level of the OB pixels as a reference.
The above-described black level is defined as described below. The level of the output signals (optical black signals) from the optical black pixel region is clamped to a predetermined target level by what is called OB clamp processing and the average value of all the optical black signals obtained with the clamp level as a reference is defined as the black level, (See Japanese Patent Application Laid-Open Publication No. 2011-55336).
In recent years, also an endoscope that employs a CMOS image sensor as an image pickup device has been proposed. Due to the influence of recent size-reduction of image pickup device chips, such a CMOS image sensor has a size limitation depending on the type thereof, and includes the optical black pixel region only in the vertical direction in the pixel array region (includes only the vertical OB pixel region).
Furthermore, in a processor adaptive to an endoscope to which a CCD image sensor is mounted as an image pickup device (hereinafter, referred to as CCD adaptive processor), OB clamp processing geared to the horizontal optical black pixel region (horizontal OB pixel region) of the CCD image sensor is generally performed.